Metal compounds, such as metal salts, and organometallic compounds are known as heat stabilizers for addition-crosslinking 2-component silicone rubber materials. EP-A-231 519 describes the use of acetylacetonates of copper, zinc, aluminum, iron, cerium, zirconium and titanium. Such metal compounds are not very compatible with the hydrophobic silicone polymer and, owing to poor miscibility, are not homogeneously distributed in the silicone rubber materials. In particular, the chelates, such as the acetylacetonates, are better distributed in the silicone rubber materials but lead to inhibition of the platinum metal complexes and consequently to a lower vulcanization rate and poor crosslinking. Colored metal compounds cause coloration of the silicone rubbers. During vulcanization and under thermal stress, discoloration and opacity occur as a result of the formation of metal hydroxides and oxides, owing to inhomogeneity.
JP-A-53058557 discloses addition-crosslinking silicone rubber materials which have, as a heat stabilizer, ferrite containing oxides of other metals. The amounts of ferrite required for heat stabilization lead to colorations and poor transparency and cannot be incorporated together with the hydrophobic filler into the silicone rubber materials since the ferrite is obtained in coarser form.
U.S. Pat. No. 4,360,388 describes the use of cerium-containing hydro-philic precipitated silica for the heat stabilization of high-temperature vulcanizing silicone rubber. U.S. Pat. No. 4,164,509 describes the imparting of hydrophobic properties to pyrogenic silica by means of titanium tetrabutylate. However, the imparting of hydro-phobic properties is not permanent; in contact with hot water, the nonwettability with water is lost. The above metal-containing silicas are not actively reinforcing fillers and thus undesirably increase the filler load of the silicone rubber without helping to improve the mechanical properties. As hydrophobic fillers, they can lead to undesirable changes in the rheological properties of the silicone rubber. The metal-containing silicas must be homo-geneously distributed in the silicone polymer in an additional dispersing step in addition to the incorporation of the reinforcing silica filler. The use of these metal-containing silicas lead to discolorations and opacity in the silicone rubber.